A typical prior art flashlight includes a battery, a light bulb and a reflector surrounding the light bulb to provide a pattern of light projected on a target object, and glass in front of the reflector. Generally, such flashlights are non-focusable and are non-adjustable in lighting range and brightness. The light pattern provided by such an arrangement is usually uneven in brightness with on and off-axis bright spots. The uneven brightness and on and off-axis bright spots are caused by imperfections in the surface of the reflector. Inconsistent light patterns are lower in overall light intensity which hampers the identification of the target object to be viewed. As a result, they typically fail to meet the working demand under some situations.
In some flashlights, the intensity of the light produced by the flashlight may be varied by moving the position of the light bulb relative to the position of the stationary reflector, thereby focusing the light between a wide beam or a narrow beam. Accordingly, some prior art flashlights include a lens in front of the reflector. The reflector and the lens are intended to collect light from the source and collimate or focus the light into a desired beam. Such light sources generally produce a diverging beam of light whereby the brightness varies across the beam. Typically, the light beam is brightest at its center, and drops off dramatically at its peripheral edge. Examples of such prior art lights may be found in U.S. Pat. Nos. 1,823,762, 2,228,078, 4,286,311, and 4,527,223.
Flashlights having a light emitting diode (LED) as the light source rather than a conventional incandescent light bulb are becoming commonplace. The LED however cannot be easily moved relative to the reflector to change the intensity.
Collimators are well known in the optical arts, and typically include a plurality of lens or reflectors that act upon light to emit nearly parallel rays. Such collimators are generally used in, for example, searchlights, headlamps and light projectors. A typical example of a light projector designed to emit a collimated beam can be found in U.S. Pat. No. 5,918,968 to Choi, which provides a parabolic reflector for converting light emitted from a lamp to parallel rays, a biconvex lens for collimating both direct and reflected light from the light source, a combination lens having a first lens and a second lens for focusing the collimated light from the biconvex lens to a focal point, and an image lens located beyond the focal point for converting the light focused at the focal point into a parallel beam.
U.S. Pat. No. 6,827,475, to Vetorino et al., combines a plurality of lens and reflectors to collimate light that includes a conical reflector disposed about the base of an LED and a lens specially designed to focus the collected light into a nearly collimated beam. The lens have opposite, substantially elliptical surfaces that collect and collimate the rapidly diverging light from the LED and the reflector. Vetorino et al., however, do not provide for the compression of the collimated beam.
Chinese patent CN201210090919.5 discloses a focus-variable flashlight having an LED light source, a moving lens, a fixed lens, a focusing cover, a cylinder body, a steel ball, a spring and the like. Both the fixed lens and the moving lens are arranged in a manner of being perpendicular to an optic axis in front of the LED light source. A constant distance is kept between the fixed lens and the LED light source, and the moving lens is capable of moving back and forth in relation to the LED light source. The lighting area and brightness of exit light can be regulated by changing the distance between the moving lens and the LED light source. This flashlight, though capable of zooming, has a relatively complex structure. The moving lens is arranged on a moving lens holder, a light transmitting lens also needs to be arranged on the head ring of the lamp cap, and during zooming, movement of the moving lens holder is driven by rotation of the focusing cover, which indispensably leads to arrangement, inside the flashlight, of a hollow cavity used for back-and-forth movement of the moving lens holder. Consequently, the flashlight has to be large in length, and becomes difficult to carry and store. Furthermore, the complex mechanism not only results in troublesome manufacturing and high cost, but is also problematic to maintenance.
U.S. Pat. Nos. 7,461,945 and 8,147,088 disclose LED flashlights incorporating a separate lens that can be moved relative to the LED to vary the intensity by changing the focus. For example, U.S. Pat. No. 7,461,945 discloses a focus-adjustable LED flashlight comprising a tubular housing; an LED light-emitting unit coupled to the tubular housing; a sleeve unit coupled movably to the tubular housing; and a positive lens mounted in the sleeve unit and spaced apart from and aligned with the light-emitting unit along an axis of the tubular housing. The sleeve unit is movable relative to the tubular housing and the light-emitting unit along the axis of the tubular housing so as to adjust a distance between the positive lens and the light-emitting unit. Specifically, the positive lens is disclosed as a plano-convex lens because such lenses show minimum spherical aberration when oriented with its plane surface facing the light source. This is in contrast to a usual symmetric convex singlet or a reversed plano-convex lens.
However, there continues to be a need for a flashlight providing a focusable high intensity collimated beam having a pronounced illuminated field with minimal halo therearound.